With an extravagant furry hood hiding her face, she ducked down and dashed at full tilt from the car into The Goring hotel – doing everything in her power to avoid being identified.

But there were several clues that the mystery woman who ran into Kate Middleton’s hotel in Belgravia yesterday afternoon was Sarah Burton, creative director of the Alexander McQueen label and very likely the wedding dress designer.

The late McQueen’s 36-year-old protégée has previously been tipped for the most coveted fashion commission of the decade.

She also owns an identical silver-studded belt, and her designs at Paris Fashion Week this year also included oversized fur hoods.

An onlooker said: ‘She ducked right down and ran in as quick as you like. The whole thing was in under half a second. It was very odd. If that’s not Sarah Burton, I’ll eat my hat.’

Kate, 29, has been determined to keep her designer and gown under wraps to ensure maximum surprise when she emerges from her Rolls-Royce outside Westminster Abbey at 11am today.

The designer is understood to be British and rumours have suggested he or she is a relatively little-known, upcoming couturier.

Sophie Cranston, Alice Temperley and Bruce Oldfield have also been mooted as possibilities.

Cheshire-born Mrs Burton was assistant and confidante to McQueen for 14 years – and was devastated by the East End designer’s suicide last year at the age of 40.

Kate is thought to have chosen Mrs Burton after being impressed by a £10,000 silk strapless wedding gown worn by her friend Sara Buys, who married the Duchess of Cornwall’s son Tom Parker Bowles in 2005.

She has also created dresses for Michelle Obama, Naomi Campbell, Lady Gaga, Gwyneth Paltrow, Cate Blanchett and Bjork.

Reports earlier this year claimed that McQueen’s chief executive Jonathan Akeroyd had told a colleague that the label had secured the commission.

Mr Akeroyd, Mrs Burton and a spokesman for the label all hurriedly insisted the story was untrue.

But Kate’s chosen designer is understood to have been sworn to secrecy, and royal aides did not deny the speculation.

One said: ‘Catherine genuinely wants to keep her dress and designer a secret until the day of the wedding, in line with tradition.

‘The details are top secret – even high-up Royal Household staff aren’t being told, especially the men.’

Kate is contributing her own design ideas to the process but if the designer is Mrs Burton – who lives in north London with her photographer husband David – we can expect dramatic tailoring, clinging fabrics and a flowing veil.